Can I fix the Pixl.js font?

You're right, fillPoly gets the rounding totally wrong. Try this:

g.drawPoly = function(p) {
  g.moveTo(p[p.length-2],p[p.length-1]);
  for (var i=0;i<p.length;i+=2)
    g.lineTo(p[i],p[i+1]);
};

function getSquare(r,x,y) {
  var poly = [];
  for (var i=0;i<4;i++) {
    var a = r + (i+0.5)*Math.PI*2/4;
    poly.push( 
        x + 20*Math.sin(a),
        y + 20*Math.cos(a)
      );
  }
  return poly;
}

g.clear();
var r = 0.02;
g.fillPoly(getSquare(r,32,32));
g.drawPoly(getSquare(r,96,32));
g.flip();

That should be pretty easily fixable, and I bet it would improve the font rendering a lot.

Edit: this is what's at fault: https://github.com/espruino/Espruino/blob/master/libs/graphics/graphics.c#L352

By adding some rounding (+128) as you suggested matters can be improved a great deal. There's still a little more magic required though - in that little test I can get 3 edges of the square rendering right, but not all - so I'd definitely appreciate some input :)

Also - thanks! This code has been around since 2014ish and nobody noticed fillPoly actually had rendering errors :)